What tools are available for monitoring on the Ops side of DevOps?

I thought that today's post by Steve Jones (We Need DevOps for Performance) was good. But I had a question that was only somewhat related to it, so I thought it better to post my question in a separate thread. I've never worked in a place that practices DevOps. And where I work now they have no intentions of ever going to DevOps. Consequently, all if my understanding is theoretical and mostly from the Dev side of DevOps. Steve's article mentioned "...regular monitoring to decide if we are over or under powered." That different tools could be used by the DBA as a part of DevOps. I've no idea if we even use any of these tools, so I would like to ask what some of these tools are. I'm just curious; want to see if we're doing any of what Steve is hinting at.